The present invention relates to automatic transmissions for land vehicles and, more particularly, to an internal bypass filtration circuit for such automatic transmissions. For purposes of this application the term “internal” refers to a physical location (i.e. a space or cavity) inside a transmission and is not be confused with terminology such as “filter with an internal by-pass”. The latter terminology refers to a common feature of a hydraulic filter element wherein if oil flow through the filter element becomes restricted, then an internal by-pass mechanism or passage within the filter opens and allows the oil to by-pass the filter element and keeps the oil circuit functioning.
Maintaining automatic transmission fluid (hereinafter “ATF”) in a clean condition free of particulates and contamination is critical to the proper function of an automatic transmission. Contamination of ATF is of particular concern to the automatic transmission rebuilding industry where there is a general trend toward achieving increased service longevity from remanufactured automatic transmissions. However, recent industry data is confirming exponentially higher contamination levels in remanufactured units due to various contributing factors and, accordingly, there is a need for increased filtration efficiency in such remanufactured transmissions.
Most automatic transmissions and transaxles use a standard wet sump system wherein ATF drains by gravity into the pan and the pump takes it up directly and discharges it to the hydraulic system. In a typical automotive application an ATF sump filter is submerged in the pan in fluid communication with the transmission pump. As the pump rotates ATF is drawn from the sump and through the filter by vacuum into the suction port of the pump.
Pumps in automatic transmission hydraulic systems are typically positive displacement pumps driven at engine speed. A positive displacement pump is one, which has the same output per revolution regardless of pump speed or pressure already developed in the system. Given the continuous flow requirements of the hydraulic system, a consistent flow of ATF through the filter to the pump is critical to proper lubrication and function of the transmission.
Because a sump filter is on the inlet side of the pump, it must not unduly restrict ATF uptake to the pump. This is a particular problem during cold start-up, hot idle and high volume/demand situations. Thus, it will be appreciated by those skilled in the art that the ATF filtration system has conflicting functions (i.e. ATF must flow freely to the pump without restriction, but must be filtered to remove harmful contaminants, which inherently restricts ATF flow).
The present internal bypass filtration system has been developed to resolve this problem and other shortcomings of the prior art.